1. Field of the Invention
This invention pertains to leaky wave antennas particularly dielectric trough waveguide antennas for use at millimeter wave frequencies.
2. Description of the Prior Art
The designs of most antennas that operate at millimeter wavelength frequencies have evolved from optical or quasi-optical concepts for focusing beams. These concepts have included parabolic reflectors, lenses and horns which are used to obtain a pencil beam. However, when they are used in a seeker radar or radiometer mounted in a missile, these designs have the limitations of relatively large volume and require mechanical movement to scan the beam. The depth and overall volume of optical type antennas can be reduced by using planar arrays. There are a large variety of planar arrays that have compact feed systems and thin overall depth. A survey of such arrays is given by P. S. Hall and J. R. James, in "Survey of Design Techniques for Flat Profile Microwave Antennas and Arrays", The Radio and Electronic Engineer, Vol. 48, No. 11, pp. 549-565, November 1978.
One type of planar array is a slotted waveguide composed of discrete radiators on a non-radiating transmission line. In addition to the slotted waveguide, a variety of leaky wave antennas with periodic radiators are well known in the art. There are dielectric image lines and dielectric waveguides with gratings composed of metal patches along the side to couple energy out of the waveguide to form a beam. Such antennas are described by
T. Itoh, "Application of Gratings in a Dielectric Waveguide for Leaky-Wave Antennas and Band-Reject Filters", IEEE Transactions MTT-25, No. 12, December 1977, pp. 1134-1138. PA0 K. L. Klohn et al, "Silicon Waveguide Frequency Scanning Linear Array Antenna", IEEE Transactions, MTT-26, No. 10, October, 1978, pp. 764-773. PA0 K. Solbach, "E-Band Leaky Wave Antenna Using Dielectric Image Line with Etched Radiating Elements", IEEE, MTT International Microwave Symposium, Orlando, FL. 1979. PA0 M. T. Birand et al, "A Printed Millimeter Wave Array Using a Low Loss Dielectric Waveguide Feeder", IEE-AP Conference, York, England, April 1981. PA0 S. T. Peng, and A. A. Oliner, "Radiation from Grating Antennas on Dielectric Waveguides of Finite Width", European Microwave Conference, Amsterdam, The Netherlands, September, 1981 pp. 757-762,
There is some reliable analytic evidence reported by,
that the grating described above can excite significant crosspolarization in the radiated field. A potentially more serious problem is the mutual coupling between the leaky wave elements in the planar array. Each of the leaky wave elements produce a fan beam with significant radiation that can couple into adjacent elements. There are other types of leaky wave antennas that might be considered for use in planar arrays including the metallic trough waveguide antenna and the sandwich wire antenna. The metallic trough waveguide antenna does not have a dielectric substrate which is advantageous to make the device compatible with integrated circuits. The sandwich wire antenna is more difficult to fabricant and has potentially higher loss because of the thin wire center conductor. Previous planar antennas all have certain undesirable features such as high ohmic loss, manufacturing difficulty, incompatibility with electronic steering, narrow operating bandwidth or limited frequency range. The present invention overcomes the above mentioned limitations at millimeter wave frequencies through the use of a dielectric trough waveguide antenna having a plurality of metallic radiators spaced periodically along the top surface of the dielectric. In the aperture of the antenna, the E-field goes to zero at the side wall. This constraint and the polarization minimize the coupling into the adjacent trough waveguide antenna.